Rotor and electrical generator

ABSTRACT

A rotor for an electrical generator and a generator incorporating such a rotor, the rotor consisting of a central hub, a radially spaced concentric rim portion with rotor magnetic elements mounted upon it, and a plurality of elongate tension members extending generally between the hub and the rim, maintained substantially in tension so as to maintain the rim substantially in compression. The hub is rotatably driven by an axially spaced power drive means such as a wind or marine current turbine.

This is a nationalization of PCT/GB02/02288 filed Jun. 5, 2002 andpublished in English.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a rotor for an electrical generator and to anelectrical generator incorporating such a rotor.

In particular the invention relates to a rotor for a low speed generatorand to a low speed generator incorporating such a rotor, that is to saya generator in which a driving force is applied to a power means, and inparticular a rotating power means such as a turbine, which ismechanically coupled to cause rotation of a rotor either by direct driveor via a low-ratio gearing.

The invention relates especially to the provision of large scale windturbine generators and marine current turbine generators, and the priorart is discussed and examples of the invention are given in the contextof the former in particular. However, it will be understood that theinvention finds potential application generally in electrical generatorswhere rotors experience high torque.

2. Description of Related Art

Most of the first commercial wind turbines used electrical generatorsthat were adapted from general purpose induction motors. It was commonpractice to connect the turbine rotor shaft rotating at typically 50 rpmor less via a multi-stage gearbox providing high ratio gearing to agenerator typically rotating at approximately 1500 rpm. However,multi-stage gearboxes are complex, heavy, costly, require maintenanceand there have been reliability issues in wind turbine applications.

As the wind power industry has grown and the typical power ratings ofturbines has been increased, new generator types have been developed tomeet the specific demands of the sector. Recently, direct-drivegenerators, which eliminate the gearbox entirely, have emerged andcaptured a significant proportion of the market. These generators aredesigned to provide the required electrical output whilst rotating atthe same speed as the turbine, thereby making the gearbox unnecessary.This simplifies the mechanical arrangement, potentially reducing costsand maintenance outage times.

In conventional direct drive generators of typical design, a largediameter bladed turbine, typically two or three blades, is axiallyspaced from and directly coupled via the turbine rotor shaft to asmaller diameter rotor. Direct-drive generators need to have a largediameter to compensate for the low rotational speed of the turbine andretain adequate peripheral speed of the generator rotor. Often the axiallength of the generator is quite small. Direct-drive generators aretherefore characterised by a disc-like appearance.

There is a general desire to introduce larger turbines to increasecapacity and efficiency, particularly in relation to the introduction oflarge offshore wind farms. Existing direct-drive turbines will becomeimpractical if simply scaled up from existing designs because ofproblems associated with manufacture, weight and transportation ofcomponents of such extreme dimensions.

Wind turbines of 2 MW rating are now available commercially and muchlarger generators are envisaged, mainly for the emerging offshoremarket. However, for example, a 5 MW direct drive generator would needto be approximately 15 meters in diameter and would be extremely heavy.

There are several problem areas arising from the size and weight ofthese generators. These include:

-   (i) the need for very large bore machine tools-   (ii) high roofed workshops with large capacity cranes are needed-   (iii) transportation of large out-sourced parts for assembly, or the    finished generator. This is even more problematic for the export    market-   (v) installation at the tower head-   (vi) tower head weight impact on other aspects of the wind turbine    design

U.S. Pat. No. 6,064,123 describes an alternative approach for windturbine generators to the typical design of a larger diameter bladedturbine axially spaced from a smaller diameter rotor with or without agearbox in between. In U.S. Pat. No. 6,064,123, the several large windturbine blades which are normally used to extract power from the windare replaced by a rotatably mounted central hub, a rim concentric withthe hub and a plurality of blades disposed between the hub and the rim.The rotor of the generator is incorporated into this turbine structureby having a plurality of magnets disposed on the rim for generatingcurrent in the stator. This structure means that the rotor rim diameteris similar to the diameter of the blade arrangement.

Whilst this structure gives high peripheral speed relative to anequivalent conventional direct drive design, with the associatedbenefits of high peripheral speed, the extremely large structure withcombined purpose exacerbates some of the above problems and createsproblems of its own, and as a result fails to exploit or allow many ofthe other benefits identified with conventional direct drive design (forexample in the use of two or three blades).

The heavy iron cores associated with conventional design direct drivegenerators also serve to exacerbate problems, in particular in relationto weight, efficiency losses, magnetic forces during assembly and airgap forces.

As the skilled person will appreciate, many of the above problems alsoarise in relation to other situations where large rotors, and inparticular large disc rotors, are conventionally employed. Inparticular, similar problems arise in relation to the design of marinecurrent turbine generators and the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a rotor for anelectrical generator which has a structure such as to mitigate some orall of the above disadvantages, and an electrical generatorincorporating such a rotor.

It is a particular object of the present invention to provide a rotor,and especially a rotor for a low speed generator driven directly or viaa low-ratio gearing by a wind or marine current turbine, which can beeconomically fabricated on a large scale.

It is a particular object of the present invention to provide a rotorfor an electrical generator, and an electrical generator incorporatingsuch a rotor, which offers improved size to weight and/or power toweight performance.

Thus, according to a first aspect of the invention in its broadestconcept, a rotor for an electrical generator comprises a central hub, arim radially spaced therefrom and generally concentric therewith, and aplurality of elongate tension members extending generally between thehub and the rim, maintained substantially in tension so as to maintainthe rim substantially in compression, and a plurality of magneticelements mounted thereon disposed around substantially the entire rim atgenerally even spacing, the hub being mountable to be rotatably drivenby a remote, and in particular an axially spaced turbine.

The rigidity given to the structure in accordance with the invention bya rim maintained in compression and generally radial struts or spokesmaintained in tension is sufficient for the rotor and conferssignificant weight reduction compared with conventional approaches forrotors.

The structure thus lends itself to construction of large scale rotorsfor application in low speed electrical generators, where the rotor iseither directly driven or driven through a low-ratio gearbox, and forexample a single stage gearbox. The previously mentioned problemsinvolved in the provision of multi-stage high ratio gearings in theprior art can be dispensed with. The structure thus lends itself inparticular to rotors which serve as an alternative to the large discrotors conventionally employed in direct-drive generators such as windor marine current turbines.

The structure can be employed in conjunction with any suitable design oflow speed and in particular direct drive wind or marine current turbinesconsisting of power drives in the form of large diameter bladed turbineswhich are axially spaced from and operatively coupled via the turbinerotor shaft to the hub of the rotor in a conventional manner, either viaa low ratio for example single stage gearbox or more preferably directlyvia an ungeared mechanical coupling. Typical applications are describedwith reference to horizontal axis turbines, but the invention is not solimited, being applicable in principle to vertical axis turbines or anyother practical turbine arrangement where the advantages it offers mightbe relevant.

The structure contrasts with the alternative proposed in U.S. Pat. No.6,064,123 which has a circular rotor/turbine arrangement but in whichthe generator mechanism is integral with the turbine blades. This meansthat the rotor rim diameter is similar to the diameter of the bladearrangement, so that the system fails to exploit or allow many of theother benefits identified with conventional direct drive design (forexample in the use of two or three blades). By contrast, the presentinvention admits these benefits in admirable manner and describes arotor structure which does not incorporate blades or any other means toextract power from the wind or water. In particular blades are notmounted on the tension members. It is solely a rotor (or in combinationwith stator and electrical take off a generator) adapted to be used inconjunction with any other suitable power drive means such as a bladedturbine to extract power from the wind or water flow.

In accordance with the invention, at least one magnetic rotor element isdisposed on the rim. The magnetic elements may be disposed on an inneror outer surface of the rim. The rotor comprises a plurality of magneticelements disposed substantially fully around the rim, and generallyevenly spaced thereon. These may be in the form of a plurality ofdiscrete magnets, or in the form of differently polarized zones in alarger magnet, or in the form of coils or in some combination thereof.

The plurality of magnetic elements are suitably located and polarised tocreate a multipolar distribution of magnetic flux in the spacesurrounding the rotor. For example the magnets may be disposed along therim on an outer or inner surface of the rim and polarised radially,axially, or a combination of both, with a resulting flux distributioncontaining radial and/or tangential and/or axial components.

The structural rim of the rotor may be circular for simplicity, or maybe polygonal, and in particular though not necessarily polygonal havinga large number of sides to approximate to circularity. References hereinto circumferential aspects of the rim, or to an arc of the rim, or to aradial direction should be construed accordingly as including referencesto the perimeter of such a polygon or a portion thereof or a directionfrom centre to edge thereof as the case may be.

The rim may be of unitary construction, or may be of modularconstruction, comprising a plurality of arc sections or sides of apolygonal rim as the case may be. The rim may be of solid construction,or of open frame construction for example in the form of a truss or likestructure, provide such structure is suitably constructed to allow therim to be maintained substantially in compression in accordance with theprinciples of the invention.

The tension members extend generally radially from the hub to the rim.Preferably however, the tension members are not directly radial, but areaxially offset and tangentially sloping. This arrangement will befamiliar, and will be understood to provide improved transmission oftorque within the structure when it is rotating. Preferably,tangentially sloping tension members are used so disposed as to resisttorque in either direction. The arrangement may be asymmetric so as toresist torque more in one direction than the other. The whole structurebeing prestressed, however, is more conveniently designed to besymmetrical and torque is transmitted by virtue of increased tension inthe spokes that slope tangentially in one direction and reduced tensionin spokes that slope tangentially in the opposite direction.

The tension members are arranged in a radial or tangential patterngenerally in the plane of rotation. However, the tension members arepreferably sloping to some extent relative to the plane of rotation inan axial direction, allowing the structure to resist axial forces.

Preferably, the rotor magnetic elements comprise a plurality of magnets,and in particular a plurality of magnets in alternate multi-polararrangement, which are preferably disposed at substantially even spacingaround one surface and in particular an outer surface of the rim.Alternatively, the plurality of magnets are arranged with identicalpolarisation to create an array of magnetic poles of one polarity withintermediate consequent poles of opposite polarity.

Preferably the magnets are permanent magnets of a material exhibitingvery high coercive force. Suitable materials will suggest themselves tothose skilled in the art, and include high coercive force ferrite,sintered neodymium-iron-boron, iron-boron and the like. Additionally oralternatively, magnets may be provided as non-permanent magnets, forexample as wound coils.

The magnets may be attached to the rim in any suitable manner forexample glued or fitted into recesses in the rim. In this latter case,magnetic forces contribute to retention, and may be sufficient alone.

The rim needs to exhibit adequate structural properties in compressionbut preferably also comprises ferromagnetic material. Preferably, therim is fabricated from a ferromagnetic material exhibiting goodproperties in compression, such as a magnetic steel. Additionally oralternatively, the rim may comprise a structural portion fabricated frommaterial selected for strength in compression, and a magnetic elementmounting portion attached thereto to provide a mounting for the magneticelements, and comprising a material having suitable ferromagneticproperties.

The elongate tension members comprise struts, spokes or the like, andare in the form of material strong in tension assembled as a wire, cableor rods to be used in tension. The tension members are of any materialsusceptible of being pre-stressed and maintained in tension in thefinished structure, so as to maintain the rim in compression forrigidity. Suitable materials include steel, glass fibre, carbon fibre,kevlar or other high strength fibre. The hub is of any suitablestructural material, such as steel.

The invention provides a physical structure for a direct-drive generatorof large diameter unrestricted by the constraints listed earlier.

In accordance with a second aspect of the invention, an electricalgenerator comprises a rotor as hereinbefore described. In particular, anelectrical generator in accordance with the invention comprises a rotoras hereinbefore described, and a suitable stator arrangement. The rotorand stator may be disposed for any flux distribution.

Accordingly, the generator comprises at least one rotor in accordancewith the first aspect of the invention co-operatively arranged with atleast one suitable stator such as to generate electrical energy frommechanical energy attributable to rotation of the rotor, the rotor beingmountable to be rotatably driven relative to a statically mounted statorby a remote, and in particular an axially spaced, turbine.

The turbine provides the driving power for the rotor and comprises anysuitable drive means in which power is produced by the action of a fluidincident thereon. The turbine is in use operatively coupled to the rotorto cause it to rotate. This may be directly or indirectly throughsuitable gearing or other transmission system.

Preferably, the electrical generator is a low speed generator in which adriving force is supplied from a driven power means, and in particular arotating power means, which is operatively mechanically coupled to thehub to cause rotation of a rotor via a low ratio gearing, for example ata ratio between 1:1 and 1:10, which may be in the form of a single stagegearbox. More preferably, the electrical generator is a direct-drivegenerator in which a driving force is supplied from a driven powermeans, and in particular a rotating power means, which is directlymechanically coupled to the hub to cause rotation of a rotor (i.e. at1:1 gearing). The invention relates in particular to the provision oflarge scale wind turbine generators and marine current turbinegenerators.

An alternative means of coupling comprises one or more flexible tensionmembers connected between the rotor rim and each blade of the turbine,for example at positions intermediate between the turbine hub andturbine blade tip. By this means force exerted by the blades istransferred to the rotor without imposing high bending moments in theblades near their attachment to the turbine hub. The tension members inthe generator rotor structure are then not required to transmit torque,so are less highly stressed in service. The power drive means is a fluiddriven turbine adapted to be caused to rotate by a stream of fluidincident thereon, and operatively coupled to the rotor such thatmovement of the fluid driven means causes rotation of the rotor. Inparticular, the fluid driven means comprises a fluid driven rotator,which is rotatable under the action of incident fluid. Preferably, thefluid driven rotator is a wind or marine current turbine.

In the preferred embodiment, the generator is a directly drivengenerator, so that the wind or marine current turbine or other fluiddriven rotator is directly coupled to the rotor such that rotation ofthe fluid driven rotator imparts an equal rotation to the rotor. Anysuitable conventional structure may be employed. For example, therotator may comprise a plurality of blades rotatable under action of theincident fluid mounted on a central axis.

Preferably, the turbine or other fluid driven rotator is coaxial withthe rotor, particularly where the rotor is directly driven.

In a preferred embodiment, the stator is configured to be seated in thevicinity of the said rim for at least an arc thereof and possibly forsubstantially all the circumference thereof. In a particular preferredembodiment, the stator circumferentially surrounds the rotor for thesaid arc or circumference, although an alternative structure in whichthe rotor circumferentially surrounds the stator can be envisaged.

Preferably, both rotor and stator extend circumferentially aroundconcentric support structures, for substantially all the circumferenceor at least an arc thereof in the case of the stator. In particular, thestator circumferentially surrounds the rotor, although an alternativestructure in which the rotor circumferentially surrounds the stator canbe envisaged.

In a particularly preferred embodiment, a stator support structure isprovided which circumferentially surrounds the rotor rim. In particular,the support structure supports a plurality of stator coils disposed toform a cylinder surrounding and coaxial with the rotor and the axis ofeach coil is directed towards the axis of the rotor. The rotor comprisesa plurality of permanent magnets disposed circumferentially around therotor rim and polarised radially to create a multi-polar distribution ofmagnetic flux, and the stator structure supports a set of coilssimilarly directed radially, such that, as the rotor turns, the fluxlinkage with each coil changes cyclically and an alternating emf isinduced in each coil. In the alternative, the rotor rim maycircumferentially surround the stator support, magnets being disposedinwardly on the former and stator coils outwardly on the latter.

In the preferred embodiment as described above, the stator coils aredisposed circumferentially around the rotor on a support structure. Anysuitable stator support structure may be envisaged. Conveniently, thestator support structure may comprise a similar open structure to thatof the rotor, that is a central hub, a circular or possibly polygonalrim, and radially extending tension members extending therebetween andmaintained substantially in tension to maintain the rim substantially incompression. Rotor and stator supports may then be coaxially mounted toprovide a suitable concentric and relatively rotatable rotor and statorarrangement.

A ferromagnetic stator core may be provided to enhance the flux providedby the permanent magnets, as is conventional in prior art systems,including direct-drive generators for wind turbines. However, the rotorand the stator core then experience a strong magnetic attractive force.Normally the magnetic attractive force is the largest force imposed onthe rotor and stator structure as assembled and their structural designis determined by the need to withstand this force. If the stator core isomitted, the attractive force is absent and a much lighter structure canbe used. Surprisingly, this is found to give quite adequateelectromagnetic performance, the relatively low flux density in theironless stator being more than compensated by the relatively highrelative speed of the magnets at the rim of the large diameter rotor.

In one embodiment of the invention therefore, the stator arrangement iscoreless; that is, no iron core is provided on the stator. Inalternative embodiments, a conventional iron core may remain desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Several alternative electromagnetic topologies may be employed and willreadily suggest themselves to those skilled in the art. The invention isdescribed with regard to its application with one particular examplewith reference to FIGS. 1 to 3 of the accompanying drawings.

FIG. 1 depicts an electromagnetic configuration of a rotor and stator.

FIG. 2( i) shows a cantilevered stator. FIG. 2 (ii) shows a double sidedstator. FIG. 2 (iii) shows a stator with rigid supports.

FIG. 3 shows a rotor and stator structure in accordance with the firstarrangement of FIG. 2 is illustrated both in exploded (FIG. 3 a) and inassembled (FIG. 3 b) view.

The examples all employ radially polarised magnets, dispense with aniron core on the stator, and employ the structure of the invention forboth rotor and stator. It will be readily understood however that theinvention is equally applicable in principle to other magneticgeometries, and/or to iron cored or coreless stator arrangements, andthat a rotor in accordance with the structure of the invention could beused in association with any suitable stator, such as one extendingcircumferentially for only an arc around the rotor.

DETAILED DESCRIPTION OF THE INVENTION

A preferred electromagnetic configuration is as shown in FIG. 1. In thisexample a ferromagnetic cylinder (1) makes up the rotor back iron andcarries neodymium-iron-boron permanent magnets (2) on its outer surface.These may be fixed in any suitable manner, for example using adhesive,but the magnetic forces alone might be sufficient to retain the magnetsin position. The magnets are polarised radially and create a multipolardistribution of magnetic flux in the space surrounding the rotor. Thestator structure supports a set of copper coils (3), whose axes aredirected radially, on a non magnetic backing (not shown). Adhesive orother fixing may be used to fix them in place. Coils may be embedded inthe support structure. This will provide good electrical insulation.

As the rotor turns the flux linkage with each coil changes cyclicallyand alternating emf is induced in each coil. In contrast to prior artsystems, no iron core is provided for the reasons set out above.

Conventionally, a ferromagnetic stator core is generally provided toenhance the flux provided by the permanent magnets. However, it has beenfound in accordance with the invention that if permanent magnets ofsufficient coercive force are used, this enhancement is unnecessary, andadequate flux linkage occurs even without a core. Omission of the coregreatly reduces the magnetic attractive force experienced by thestructure so that a much lighter structure can be used and theadvantages of the open structure of the present invention can beexploited to the full. For this reason, the magnets (2) are fabricatedfrom sintered neodymium-iron-boron to provide the necessary coerciveforces.

In the preferred embodiment, both the stator support structure and therotor structure are based on the same open principles of rim incompression with radially extending tension members. Three possiblearrangements are shown in FIG. 2.

In each case like references are used for like components. A centralshaft (6) with centre line (7) is shown on which the hub (8) is mountedfor rotation in direction of the arrow. Rotor spoking (11) extends to arotor rim (12). Stator spoking (14) extends to a stator rim (15)concentic with the rotor rim (12). An air gap (17) is provided betweenthe rotor rim (12) and stator rim (15). The designs differ in overallarrangement of these common components, and also in that 2(ii) isprovided with a bearing (18) and 2(iii) incorporates rigid statorsupports (19) which are maintained in compression.

In FIG. 3, the rotor support structure is shown comprising a circularrim (21) maintained in compression by spokes (22) connected to a centralhub (23). Permanent magnets (not shown) are mounted on the rim (21) inthe manner indicated in FIG. 1.

A stator arrangement is shown comprising a pair of rims (24 a), (24 b)linked by cross members (25), and maintained in compression by spokes(26) in tension extending to a central hub (27). Coils (28) are mountedon an inner surface of the first rim (24 a).

This is just one example of structure and other structures will suggestthemselves.

In most preferred structures, both stator structure and rotor structureof the generator are based on one or more outer circular (or possiblypolygonal) members acting in compression, which are connected to thecentral hub/wind turbine rotor arrangement by circular arrays of membersacting in tension rather like the spokes of a bicycle wheel. Theembodiments illustrate this. The tension members are arranged in aradial or tangential pattern in the plane of rotation, and are slopingin the axial direction, thus allowing the stator and rotor structures toresist torque and any axial forces. When tangentially sloping tensionmembers are used, these can be arranged to resist torque in eitherdirection. In some embodiments the plane containing the rim may not liebetween the planes containing the rings at which the spokes are securedto the hub. In these cases it is necessary to replace one set of tensionmembers by compression members.

Other arrangements applying the principles of the invention will bereadily envisaged.

1. An electrical generator comprising: a rotor comprising a central hub,a rim radially spaced therefrom and generally concentric therewith, anda plurality of elongate tension members extending generally between thehub and the rim, said tension members pre-stressed to be maintained intension so as to maintain the rim substantially in compression, and aplurality of magnetic elements disposed substantially fully around therim and generally evenly spaced thereon; at least one statorco-operatively arranged relative to said rotor such as to generateelectrical energy from mechanical energy attributable to rotation of therotor, wherein said stator comprises a central hub, a rim radiallyspaced therefrom, and a plurality of tension members maintained intension extending generally between the hub and the rim so as tomaintain the rim substantially in compression; and a turbine remotelyspaced from said rotor, said turbine comprising a wind or marine currentturbine; wherein the rotor is mounted to be rotatably driven relative tosaid statically mounted stator by the remotely spaced turbine.
 2. Anelectrical generator in accordance with claim 1 wherein the plurality ofmagnetic elements of the rotor are disposed around the rim thereof andpolarised radially.
 3. An electrical generator in accordance with claim1 wherein the plurality of magnetic elements of the rotor are disposedaround the rim thereof and polarised axially.
 4. An electrical generatorin accordance with claim 1 wherein the rotor magnetic elements comprisea plurality of permanent magnets.
 5. An electrical generator inaccordance with claim 1 wherein the stator comprises a plurality ofwound coils.
 6. An electrical generator in accordance with claim 5wherein the stator coils are provided in association with one or moreferromagnetic stator cores to enhance the flux provided with themagnetic elements.
 7. An electrical generator in accordance with claim 1wherein the tension members of the rotor extend generally radially fromthe hub to the rim but are axially offset and tangentially sloping toresist axial forces and torque.
 8. An electrical generator in accordancewith claim 1 wherein the stator is configured to be seated in thevicinity of the rim of the rotor for at least an arc thereof.
 9. Anelectrical generator in accordance with claim 8 wherein the stator isconfigured to be seated in the vicinity of the said rim forsubstantially all the circumference thereof.
 10. An electrical generatorin accordance with claim 9 wherein the stator coils are disposedcircumferentially around the rotor rim, the rotor and stator supportsbeing coaxially mounted to provide a concentric and relatively rotatablerotor and stator arrangement.
 11. An electrical generator in accordancewith claim 10 wherein the stator structure fully surrounds the rotorstructure with one side of the stator structure being mounted on arotating bearing.
 12. An electrical generator in accordance with claim10 wherein the stator structure approaches the rotor structure from oneside thereof in cantilever fashion.
 13. An electrical generator inaccordance with claim 1 wherein the stator coils are provided without aferromagnetic stator core or cores.
 14. An electrical generator inaccordance with claim 1 comprising a low speed generator in which theturbine is operatively mechanically coupled to the hub to cause rotationof the rotor via a low ratio gearing.
 15. An electrical generator inaccordance with claim 1 comprising a direct drive generator in which theturbine is directly mechanically coupled to the hub to cause rotation ofthe rotor.
 16. An electrical generator in accordance with claim 1comprising a direct drive generator in which the turbine is directlymechanically coupled to the rim to cause rotation of the rotor.
 17. Anelectrical generator in accordance with claim 1 wherein said turbineforms no part of said rotor.
 18. An electrical generator in accordancewith claim 1 wherein said rotor hub, rotor rim, and elongate tensionmembers extending therebetween are rotatable in a circular path and saidturbine is axially separated from said circular path.
 19. An electricalgenerator as claimed in claim 1 wherein said elongate tension membersare free of any turbine components.